1. Field of the Invention
The present invention relates to a gas laser having a frequency-selective multiple layer system formed on a Brewster window of the laser discharge tube.
2. Description of the Prior Art
It is known in the art to provide means for suppressing a single wavelength of the plurality of wavelengths usually emitted by a gas laser. For example, German Pat. OS No. 23 42 911 discloses coating one of two resonator mirrors (in a gas laser) wherein the coating includes a .lambda./2 wavelength layer and symmetrically disposed on either side thereof a plurality of pairs of alternating optically dense and optically thin .lambda./4 wavelength layers, where .lambda. is the wavelength of the unwanted light. Such a layer system yields a pronounced reflection minimum. A defined wavelength, or line, can be filtered out from a series of laser lines that are in relatively close proximity. However, suppressing all but one line of a spectrum of lines with such layer system does present problems.
A modified coating could be used to obtain suppression of all but one frequency but the reflection maximums of such layer structures as are required for such suppression are temperature and pressure dependent to such an extent as to result in a noticeable loss of power such as, for example, in an Ar.sup.+ laser which has relatively close wavelength spacings. By applying dielectric cut-off filters to both mirrors the conditions for wavelength selection are less critical, although greater costs and more complex manufacturing are involved since the soft material of the mirror surface, which is frequently curved, must be polished and coated.
It is also known to use etalons, frequency-selective polarizers or dispersing elements for line selection (see German Pat. OS. No. 20 60 525 or GB OS No. 20 75 746), in conjunction with broad band mirrors. Such additional elements in the resonator however, attenuate the output beam and make the structure more complicated.